Etymologies

Middle English avouchen, to cite as a warrant, from Old French avochier, from Latin advocāre, to summon; see advocate.

(American Heritage® Dictionary of the English Language, Fourth Edition)

From Old French avouchier, from Latin advocāre, present active infinitive of advocō. (Wiktionary)

Examples

The Duke had now recovered his breath, and was able to sit up and give attention to what passed betwixt Dunois and Crawford, while the former pleaded eagerly that there was no occasion to mention in the matter the name of the most noble Orleans, while he was ready to take the whole blame on his own shoulders, and to avouch that the Duke had only come thither in friendship to him.

They avouch that every element and different state of being have animals resembling those of another element, as there be fishes at sea resembling Monks of late order in all their hoods and dresses, so as the Roman invention of good and bad daemons and guardian angels particularly assigned, is called by them ane ignorant mistake, springing only from this originall.

“I will be open with you, my father — bid these men stand out of ear-shot, and I will tell you all I know of this mysterious business; and muse not, good father, though it may pass thy wit to expound it, for I avouch to you it is too dark for mine own.”

Then Affan and Bulukiya fared on for the hills where grew the herbs; and, as they went about with the Queen, each plant they passed began to speak and avouch its virtues by permission of Allah the Most High.

Fear makes our imagination conceive what it list, invites the devil to come to us, as [1667] Agrippa and Cardan avouch, and tyranniseth over our phantasy more than all other affections, especially in the dark.